In a large saucepan, whisk together 1 1/2 cups (375 ml) of the water with the sugar, cocoa powder, and salt. Bring to a boil whisking frequently. Let it boil, continuing to whisk for 45 seconds.

Remove from the heat and stir in the chocolate until it's melted, then stir in the vanilla extract and the remaining 3/4 cup (180 ml) water. Transfer the mixture to a blender and blend for 15 seconds. Chill the mixture thoroughly, then freeze it in your ice cream maker according to the manufacturer's instructions. If the mixture has become too thick to pour into your machine, whisk it vigorously to thin it out.